Kajian pengaruh peningkatan pH tanah Podsolik merah-kuning atas pengambilan fosfor dari batuan fosfat oleh padi gogo (Oryza sativa L.)=A Study on the Effect of Increasing pH of Red-Yellow Podzolicby Upland Rice (Oryza sa

ABSTRACT A pot experiment to study the effect of direct application of phosphate rock on P uptake by upland rice variety IR 36 on a Red- Yellow Podzolic Soil with increased pH (from 4.96 to 5.86) has been carried out in the greenhouse by using TSP as a reference fertilizer. The results showed that application of phosphate rock at increasing soli pH from 4.96 to 6.45 substantially increased plant height at 45 and 60 days after planting (DAP), and dry weight of tops at the maximum vegetative growth stage (63 DAP). Soil extractable P and uptake of P in tops at harvest, increased when the soil pH was increased from 4.96 to 6.86. The application of phosphate rock at each pH level did not have any significant effect on the height of plants at 15, 30, and 45 DAP, and dry weight of tops, up to the highest rate (800 kg equivalent to TSP/ha) tested. Soil extractable P and uptake of P in tops at harvest from the highest rate of phosphate rock did not show any significant change. In contrast, the application of TSP was comparably superior in its effects at equivalent rates. The relative agronomic effectiveness value o f phosphate rock compared to TSP was very low at each soil pH level, and the value tended to decrease at pH 5.86 The results of this experiment implied that the phosphate rock used (Jordan type) had a very low solubility, and presumably required a certain incubation period (at least 60 days) to supply an adequate P for the initial growth of upland rice variety rR 36. The very low solubility of the Jordan phosphate rock made accurate interpretation on the uptake of P by upland rice difficult. Inherent properties of phosphate rack sources, soil properties, and crop species must be considered in order to obtain desired effects from direct application of phosphate rock. Key words: podzolic soil - phosphate rock - upland rice TSP - relative agronomic effectivenes